The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Some radio frequency (RF) transmitters require accurate control of transmitted output power. For example, many RF transmitters need to comply with FCC regulations and wireless standards. Control of output power can be accomplished using an open loop or closed loop control system. In open loop control systems, the RF transmitter relies on accurate gain steps within the transmitter. In closed loop control systems, output power is measured and gain is adjusted accordingly.
An RF power detection circuit is an integral part of any RF transmitter closed-loop power-control system. The RF power detection circuit measures absolute transmitted power. This measurement is preferably independent of variation in temperature, device characteristics due to process spread, and load/antenna impedance.
Some RF power detection circuits assume a resistance value of an output load such as an antenna, measure output voltage and calculate output power based the output voltage squared divided by the resistance value. However, the resistance value of the load such as the antenna may vary during operation. For example, the resistance value of the antenna may be affected when the antenna is near or comes in contact with other objects. As can be appreciated, the RF power calculation will be adversely affected due to the difference between the actual resistance value of the antenna and the assumed resistance value.